Circularly polarized (CP) light emitting devices have enormous potential in a wide range of applications, including highly efficient LCD backlights, stereoscopic 3D displays, optical quantum information processing and communication. Polymer light emitting diodes (PLEDs) are devices that directly generate electroluminescence (EL) from an ultra-thin semiconducting polymer layer and have many advantages as a light source, including solution processibility, large area coverage, and flexibility. So far, attempts at generating circularly polarized electroluminescence (CP-EL) from PLEDs have involved either complex and bulky device architectures or custom synthesized non-standard polymers.
The first successful attempt to generate CP-EL directly from a PLED used a chiral-substituted poly(p-phenylene vinylene) (PPV) derivative (Peeters, E. et al. J Am Chem Soc 119, 9909-9910 (1997)). This initial PPV study only obtained g factors in the region of 10−3. A number of approaches have been taken to improve this very low degree of circular polarization. One approach is to dope luminescent achiral chromophores into chiral nematic or cholesteric liquid crystals (Chen, S. H. et al, Nature 397, 506-508 (1999) and Katsis, D., et al, Chem Mater 13, 643-647 (2001)). The linearly polarised emission generated becomes circularly polarised travelling through the birefringent chiral nematic film, and therefore although g factors are significantly improved from 10−3 to 0.3, the level of circular polarisation is strongly dependent on the film thickness. Indeed, a thickness in the region of several micro-meters was used, which is too thick for LEDs (usually tens to hundreds of nano-meters). Furthermore the LC host material is an insulator, which obviously limits application in such devices. The attachment of chiral side groups onto conductive polymer backbones other than PPV has also been explored in order to align them into chiral monodomain liquid crystalline film (Grell, M. and Bradley, D. C. C., Adv. Mater, 11, 895-905 (1999)). This technique has led to PLEDs with g factors up to 0.35, however the requirement for bespoke synthesis of a specialised polymer will likely limit its wider application. It is possible to employ wide-band reflective polarizers as passive components to generate CP light in such devices, however this complicates device architecture and limits the minimum thickness achievable. The direct generation of CP light would be far more favourable in terms of energy efficiency and product cost.